Fluorescent screen and method of making same



F. K. DAVEY Sept. 9, 1958 FLUORESCENT SCREEN Ayn METHOD OF MAKING SAME Filed Dec. 50, 1955 DIE PUNCH DIE BODY sscowo NICKEL PLATED STEEL LAYER DIE PUNCH m .D. K M

ATTORNEY United States Patent FLUORESCENT SCREEN AND METHOD OF MAKING SAME Frederick K. Davey, Baltimore, Md., assignor to Bendix Aviation Corporation, Baltimore, Md., a corporation of Delaware Application December 30, 1955, Serial No. 556,518

5 Claims. (Cl. 250-80) This invention relates to improvements in transparent screens which fluoresce under X-ray bombardment, giving oif visible light, and a method of manufacturing same. Screens of this type find extensive use for rendering visible an X-ray image of the subject undergoing study in X-ray therapeutic and diagnostic treatment, industrial X-ray work, and the like.

There are a number of factors which aifect the brightness, contrast and resolution of an image produced on a screen when subjected to X-ray bombardment. The screen must absorb X-ray energy and convert it to visible light, and the fractional amount of the incident X-ray energy so absorbed is a function of the atomic number of the elements or matter of which the screen is comprised, the thickness of the screen and the energy of the incident X-ray photons. In general, the higher the atomic number of such elements, the greater will be the absorption; and for material of given characteristics, more X-ray energy will be absorbed by a thick screen than by a thin screen. Also, more low energy X-ray photons are absorbed than are high energy photons, a given screen absorbing approximately 100 times as many 60 kv. X-ray photons as it will 1 m. e. v. photons. Sixty kv. X-rays are used for medical diagnostic work, while one m. e. v. X-rays are used for medical therapy and industrial radiographic work. Obviously, all the X-ray energy absorbed by a material is not converted to visible light, since some is dissipated as heat or to eject electrons. While a thick screen will absorb more X-ray energy than a thin screen, assuming a given type fluorescent material, the image will be brighter only if such material is transparent to the light it emits.

Among the more prominent types of fluorescent screens now being manufactured are those made of a layer of powdered fluorescent material such as zinc sulfide on a backing material such as cardboard, and those made of single crystals of fluorescent material such as sodium iodide activated by means of a small amount of thallium. Zinc sulfide screens are almost completely opaque to the light emitted and are therefore made very thin, usually about 0.015 inch. Single crystals of sodium iodide are transparent to the emitted light, and hence screens of this type can be made in thicknesses of one inch or more.

' Another and very important factor which affects the ability of the screen to produce a sharp image is freedom from scattering. By scattering is meant diffusion of the emitted light due to reflection or refraction. Grain boundaries in powdered materials such as zinc sulfide are scattering centers, and for a given density of scattered centers, the loss of resolution will be greater in thick screens. While zinc sulfide screens may be made in relatively large sizes, diametrically speaking, they are not suitable for high energy X-ray work. On the other hand, synthetic single crystals of material such as sodium iodide are available in sizes up to approximately 5 inches in diameter and 1 or '2 inches thick, but the cost of crystals of such dimensions is so great as to render their use in a commercial screen prohibitive. Attempts to H make single crystal screens of relatively large area have usually resulted in failure due to cracking and other causes.

The primary object of the present invention is to provide a fluorescent screen which may be manufactured at a relatively low cost and which at the same time will produce a bright image of good contrast and resolution.

Another object of the invention is to provide a method of manufacturing a fluorescent screen by means of which a relatively large screen may be produced at a commercially feasible cost, the screen having properties substantially similar to those possessed by a single crystal of sodium iodide.

Another object is to generally improve the quality and manufacturing technique of fluoroscopic screens of the type specified.

In the drawing:

Fig. 1 is a view in sectional perspective of a type of apparatus which may be used in making fluorescent screens in accordance with the invention; and

Fig. 2 is a perspective view of my improved screen ready for the polishing or finishing operation.

Materials which crystallize in the cubic system and which are transparent when in the form of single crystals will be substantially equally transparent in multicrystalline form if no change in the index of refraction takes place at grain boundaries. Since cubic materials are isotropic there will be no change of index of refraction within adjacent grains regardless of their relative crystallographic orientation. If there is a relatively small number of changes in the index of refraction, the scattering will be sufiicient to render the material translucent, and a substantially large number of changes in the index of refraction will render the material opaque. The amount of scattering is also a function of size or degree of change of the index of refraction; larger changes introducing more scattering. To illustrate, a light beam will not be reflected or refracted if everywhere along its path the index of refraction is the same. Furthermore, if air and other impurities are eliminated from the grain boundaries, the change of index of refraction which normally accompanies a grain boundary can be substantially eliminated.

It was discovered that the alkali halides, when treated in accordance with the method herein disclosed, fulfill the desired requirements: they crystallize in the cubic system; they can be obtained in substantially pure form and can be further purified prior to being processed, and they deform plastically at temperatures well below the melting point. (Heretofore, of the alkali halides, sodium iodide, potassium iodide and cesium iodide, all activated with small amounts of thallium have been used in single crystal form as fluorescent screens.) It has been further discovered that when powders of sodium iodide activated with thallium iodide, potassium iodide powders activated with thallium iodide, and cesium iodide powders activated with thallium iodide are subjected to pressures of from 5,000 to 20,000 p. s. i. and while under pressure are held at temperatures of from 200 to 500? C., a transparent fluoroscopic screen is produced having properties comparable to those of single crystals of sodium iodide, and furthermore, the screen can be made of comparable thickness and considerably larger in area.

The following constitutes a specific example of a method or procedure which may be followed in producing screens according to the invention:

First, make a batch of material in the following manner:

Take a given quantity of alkali halide powder in highly purified form and activate it with thallium by mixing the latter in solid form with the alkali halide. The thallium may be introduced as a salt, e. g. thallium iodide,

thallium chloride, etc. The anion of the salt is not critical but it is preferred to use the same anion as that possessed by the alkali halide; for example, use thallium iodide with potassium iodide. It is important that the alkali halide be highly purified. In practice, impurities are reduced to a ratio of one hundred parts per million parts of the halide powder, or lower.

Formation of the solid solution of the thallium salt in the alkali halide to complete activation may be carried out as by heating the mixture until molten and then cooling, or by dissolving the mixture in water and subsequently evaporating off the water.

The activated alkali halide is then granulated to a degree of fineness such that it will pass through a 100 mesh sieve. Very coarse particles may have entrapped gases, and very fine particles present an unnecessary number of grain boundaries.

The next step is to remove any moisture that may be present in the activated granular material. This may be done by subjecting the material to temperatures of from 100 to 200 C. under avacuum.

The activated material in dry granulated form is now ready for pressing into a screen of the desired dimensions. In practice, pressing equipment such as that exemplified in Fig. 1 has been used with success. For a screen of x 10 x 1'', approximately 13 pounds of the material is required. This quantity is loaded into the die cavity and subjected to a pressure of from 5,000 to 20,000 pounds per square inch for five minutes or longer, while the temperature is held substantially constant at a point between 200 and 500 C., which is below the melting temperature of the dry granules. The pressure may be applied before or after the die is heated, but it is essential that the pressure be applied for at least five minutes while the die is held at, or bordering on, some temperature between the above range.

Before attempting to eject a screen from the die, the latter should be cooled to a temperature below 100 C., which is preferably done while the pressure is still applied. Care should be taken to cool the die to a temperature where no plastic deformation of the screen can take place before ejecting the screen from the die. After ejection, the screen is ready for polishing, which may be done by any suitable means known to the art.

In practice, a die body made of stainless steel has been used successfully, heat being applied to an external electric heater. ply pressure to the die. One of the problems which it was necessary to overcome was sticking of the screen to the die face. This may be prevented by placing a piece of aluminum foil next to the material and a piece of nickel-plated steel of say 0.001 inch thick between the aluminum foil and the die face. After ejection, the steel layer will readily separate from the die face and the aluminum foil, after which the latter can be peeled from the formed screen with little difliculty. Aluminum foil will also prevent discoloration and cracking of the screen. It was further found that the screen would develop cracks if the nickel plated steel layer were thicker than approximately 0.001 inch. When aluminum foil only was used, it would tend to stick to both the die face and the screen. Should material stick to the sides of the die and an opaque border be formed around the A hydraulic press may be used to ap- 4 screen when the latter is ejected, such border will not affect the active or usable portion of the screen.

It will be obvious that certain minor variations in the constituents used in making the screen as well as the method of forming the latter may be adopted without departing from the scope of the invention as disclosed herein.

What is claimed and desired to be secured by United States Letters Patent is:

l. The method of fabricating a transparent fluorescent X-ray screen which includes the steps of activating alkali halide powder and then compressing the activated powder into the shape of the screen while maintaining the material at a temperature of at least 200 C. but less than its melting temperature. I

2. The method of fabricating a transparent fluorescent X-ray screen which includes the steps of taking a quantity of purified alkali halide powder and introducing a thallium salt activator therein, forming the resultant mixture into a solid solution, drying and granulating the solid solution, and then compressing the granules into the shape of the screen while maintaining the material at a temperature in excess of 200 C. but less than the melting temperature of the granules.

3. The method of fabricating a transparent fluorescent X-ray screen which includes the steps of taking a quantity of highly purified alkali halide powder and introducing a thallium salt activator therein, forming the mixture of alkali halide powder and thallium salt into a solid solution, drying and granulating the solid solution and then compressing the granules into a solid body by subjecting the granules to a predetermined pressure at a substantially constant temperature within a range of from 200 to 500 C.

4. The method of fabricating a transparent fluorescent X-ray screen which comprises the steps of highly purifying an alkali halide powder, introducing a thallium salt activator into the powder, forming the mixture of alkali powder and thallium salt into a solid solution, drying the solid solution and granulating the dried, solid solution to a degree of fineness where it will pass a sieve of approximately -mesh, and then subjecting the granules to a pressure of at least 5,000 p. s. i. and a substantially constant temperature within a range of from 200 to 500 C.

5. A transparent fluorescent X-ray screen comprised of activated dry granular material made up predominantly of one of the alkali halides hot-pressed into a solid cohesive body with the granules in unfused condition, said granules when in a single crystal state being'isotropic and transparent and when compressed in the form of the desired screen while at a temperature less than fusion temperature retaining substantially the same index of refraction at the grain boundaries as throughout the interior of the grains.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,960 Schwartzwalder July 5, 1938 2,569,226 Carter Sept. 25, 1951 2,689,190 Hushley Sept. 14 ,1954 2,727,863 Fonda Dec. 20, 1955 

